Changed implementation of gsl::narrow to throw gsl::narrowing_error

[londey]

Related to issue #872

Reinstated throwing behaviour of gsl::narrow described in C++ Core Guidlines ES.46

https://github.com/isocpp/CppCoreGuidelines/blob/master/CppCoreGuidelines.md#es46-avoid-lossy-narrowing-truncating-arithmetic-conversions

[JordanMaples]

I took a look at the build issues you're having.

Deleting the file no_exceptions_throw_test.cpp and removing the related line in tests/CMakeLists.txt will get everything building cleanly.

That file exists because of the old terminate vs throw behavior and is no longer needed.

[londey]

@JordanMaples #873 (comment) I wasn't sure if you wanted to keep the no-throw option for narrow and there doesn't seem to be a define to control if GSL should be no-throw.

If you are happy to remove that scenario I can remove the test.

[JordanMaples]

Thanks again for finding this issue and getting the fix ready.

[beinhaerter]

@JordanMaples I was thinking about upgrading to GSL 3.0. Seeing this fix here (and using gsl::narrow in my code) I now know that I cannot use GSL 3.0. Do you plan to release a new version of GSL like 3.1 with this fix here included?

[JordanMaples]

@beinhaerter My original plan was to release 3.1.0 when the span paper P1976R2 was implemented, but I think this warrants a minor release. I'll be pushing 3.0.1 today.

[JordanMaples]

@beinhaerter I've released 3.0.1.